atomic clock

 

electric or electronic timekeeping device that is controlled by atomic or molecular oscillations. A timekeeping device must contain or be connected to some apparatus that oscillates at a uniform rate to control the rate of movement of its hands or the rate of change of its digits. Mechanical clocks and watches use oscillating balance wheels, pendulums, and tuning forks. Much greater accuracy can be attained by using the oscillations of atoms or molecules. Because the frequency of such oscillations is so high, it is not possible to use them as a direct means of controlling a clock. Instead, the clock is controlled by a highly stable crystal oscillator whose output is automatically multiplied and compared with the frequency of the atomic system. Errors in the oscillator frequency are then automatically corrected. Time is usually displayed by an atomic clock with digital or other sophisticated readout devices.

The first atomic clock, invented in 1948, utilized the vibrations of ammonia molecules. The error between a pair of such clocks, i.e., the difference in indicated time if both were started at the same instant and later compared, was typically about one second in three thousand years. In 1955 the first cesium-beam clock (a device that uses as a reference the exact frequency of the microwave spectral line emitted by cesium atoms) was placed in operation at the National Physical Laboratory at Teddington, England. It is estimated that such a clock would gain or lose less than a second in three million years. The U.S. standard is the NIST-F1, which went into service in 1999 and should neither gain nor lose a second in 20 million years. A fountain atomic clock, the NIST F-1 consists of a 3-foot vertical tube inside a taller structure. It uses lasers to cool cesium atoms, forming a ball of atoms that lasers then toss into the air, much like one would toss a tennis ball, creating a fountain effect. This allows the atoms to be observed for much longer than could be done with any previous clock.

Many of the world's nations maintain atomic clocks at standards laboratories, the time kept by these clocks being averaged to produce a standard called international atomic time (TAI). Highly accurate time signals from these standards laboratories are broadcast around the globe by shortwave-radio broadcast stations or by artificial satellites, the signals being used for such things as tracking space vehicles, electronic navigation systems, and studying the motions of the earth's crust. The accuracy of these clocks made possible an experiment confirming an important prediction of Einstein's theory of relativity. Prototypes of atomic clocks using atoms such as hydrogen or beryllium could be still thousands of times more accurate. For example, researchers at the U.S. National Institute of Standards and Technology have demonstrated an atomic clock based on an energy transition in a single trapped mercury ion (a mercury atom that is missing one electron) that has the potential to be up to 1,000 times more accurate than current atomic clocks.

Bibliography

See F. G. Major, The Quantum Beat: The Physical Principles of Atomic Clocks (1999).

An Atomic Clock is a type of clock that uses an atomic resonance frequency standard to feed its counter. Early atomic clocks were masers with attached equipment. Today's best atomic frequency standards (or clocks) are based on absorption spectroscopy of cold atoms in atomic fountains. National standards agencies maintain an accuracy of 10^-9 seconds per day (approximately 1 part in 10¹⁴), and a precision equal to the frequency of the radio transmitter pumping the maser. The clocks maintain a continuous and stable time scale, International Atomic Time (TAI). For civil time, another time scale is disseminated, Coordinated Universal Time (UTC). UTC is derived from TAI, but synchronized with the passing of day and night based on astronomical observations.

History

The first atomic clock was built in 1949 at the U.S. National Bureau of Standards (NBS). The first accurate atomic clock, a cesium standard based on a certain transition of the cesium-133 atom, was built by Louis Essen in 1955 at the National Physical Laboratory in the UK. This led to the internationally agreed definition of the second being based on atomic time.

For decades, scientific-instrument companies, such as Hewlett-Packard, have been making cesium-fountain clocks for entities like NIST and USNO, at prices rivalling those of cars.

In August 2004, NIST scientists demonstrated a chip-scaled atomic clock. According to the researchers, the clock was believed to be one hundredth the size of any other. It was also claimed that it requires just 75 mW, making it suitable for battery-driven applications. This device could conceivably become a consumer product. It will presumably be much smaller, much less power-thirsty, and much cheaper to make than the traditional cesium-fountain clocks used by NIST and USNO as reference clocks. However, it is uncertain whether it will ever become a consumer product.

How they work

Frequency reference masers use glowing chambers of ionized gas, often cesium because that is the element used in the official international definition of the second.

Since 1967, the International System of Units (SI) has defined the second as the duration of 9 192 631 770 cycles of the radiation which corresponds to the transition between two energy levels of the ground state of the cesium-133 atom. This definition makes the cesium oscillator (often called an atomic clock) the primary standard for time and frequency measurements (see cesium standard). Other physical quantities, like the volt and metre, rely on the definition of the second as part of their own definitions. [1]

The core of the atomic clock is a tuneable microwave cavity containing the gas. In a hydrogen maser clock the gas emits microwaves (mases) on a hyperfine transition, the field in the cavity oscillates, and the cavity is tuned for maximum microwave amplitude. Alternatively, in a cesium or rubidium clock, the gas absorbs microwaves and the cavity contains an electronic amplifier to make it oscillate. For both types the atoms in the gas are prepared in one electronic state prior to filling them into the cavity. For the second type the electronic state of leaking atoms is detected and the cavity is tuned for a maximum of detected state changes.

This adjustment process is where most of the work and complexity of the clock lies. The adjustment tries to correct for unwanted side-effects, such as frequencies from other electron transitions, temperature changes, and the "spreading" in frequencies caused by ensemble effects. One way of doing this is to sweep the microwave oscillator's frequency across a narrow range to generate a modulated signal at the detector. The detector's signal can then be demodulated to apply feedback to control long-term drift in the radio frequency. In this way, the quantum-mechanical properties of the atomic transition frequency of the cesium can be used to tune the microwave oscillator to the same frequency, except for a small amount of experimental error. When a clock is first turned on, it takes a while for the oscillator to stabilize.

In practice, the feedback and monitoring mechanism is much more complex than described above.

A number of other atomic clock schemes are in use for other purposes. Rubidium standard clocks are prized for their low cost, small size (commercial standards are as small as 400 cm³), and short term stability. They are used in many commercial, portable and aerospace applications. Hydrogen masers (often manufactured in Russia) have superior short term stability to other standards, but lower long term accuracy.

Often, one standard is used to fix another. For example, some commercial applications use a Rubidium standard slaved to a GPS receiver. This achieves excellent short term accuracy, with long term accuracy equal to (and traceable to) the U.S. national time standards.

The lifetime of a standard is an important practical issue. Modern Rubidium standard tubes last more than ten years, and can cost as little as US$50. Cesium reference tubes suitable for national standards currently last about seven years and cost about US$35,000. The long-term stability of hydrogen maser standards decreases because of changes in the cavity's properties over time.

Modern clocks use magneto-optical traps to cool the atoms for boosted precision.

Application

Generating of standard frequencies. Atomic clocks are installed at each site of time signal, LORAN-C, and Alpha Navigation transmitters. They are also installed at some longwave and mediumwave broadcasting stations to deliver a very precise carrier frequency, which also has its usage as standard frequency.

Further, atomic clocks are used for long-baseline interferometry in radioastronomy.

Atomic clocks are the basis of the GPS navigation system. The GPS master clocks are Atomic clocks at the ground stations, and each of the GPS satellites has an on-board atomic clock.

Power consumption

Power consumption varies enormously, but there is a crude scaling with size. Chip scale atomic clocks can use of the order of 100 mW; NIST F1 uses orders of magnitude more power.

Research

Most research focuses on ways to make the clocks smaller, cheaper, more accurate, and more reliable. These goals often conflict.

New technologies, such as femtosecond frequency combs, optical lattices and quantum information, have enabled prototypes of next generation atomic clocks. These clocks are based on optical rather than microwave transitions. A major obstacle to developing an optical clock is the difficulty of directly measuring optical frequencies. This problem has been solved with the development of self-referenced mode-locked lasers, commonly referred to as femtosecond frequency combs. Before the demonstration of the frequency comb in 2000, terahertz techniques were needed to bridge the gap between radio and optical frequencies, and the systems for doing so were cumbersome and complicated. With the refinement of the frequency comb these measurements have become much more accessible and numerous optical clock systems are now being developed around the world.

Like in the radio range absorption spectroscopy is used to stabilize an oscillator — in this case a laser. When the optical frequency is divided down into a countable radio frequency using a femtosecond comb, the bandwidth of the phase noise is also divided by that factor. Although the bandwidth of laser phase noise is generally greater than stable microwave sources, after division it is less.

The two primary systems under consideration for use in optical frequency standards are single ions^[1] isolated in an ion trap and neutral atoms trapped in an optical lattice. These two techniques allow the atoms or ions to be highly isolated from external perturbations, thus producing an extremely stable frequency reference.

Optical clocks have already achieved better stability and lower systematic uncertainty than the best microwave clocks.^[1] This puts them in a position to replace the current standard for time, the cesium fountain clock.

Atomic systems under consideration include but are not limited to Al+, Hg+,^[1] Hg, Sr, Sr+, In+, Ca+, Ca, Yb+ and Yb.

Radio clocks

Main article: Radio clock

Modern radio clocks can be referenced to atomic clocks, and provide a way of getting high-quality atomic-derived time over a wide area using inexpensive equipment. However, radio clocks are not appropriate for high-precision scientific work. Many retailers sell radio clocks under the name "atomic clocks", but in doing so they are misrepresenting their products.

There are a number of longwave radio transmitters around the world - in particular DCF77 (Germany), HPG (Switzerland), JJY (Japan), NPL or MSF (United Kingdom), TDF (France), WWVB (United States). Many other countries can receive these signals (JJY can sometimes be received even in Western Australia and Tasmania at night), but it depends on time of day and atmospheric conditions. There is also a transit delay of approximately 1 ms for every 300 km the receiver is from the transmitter. When operating properly and when correctly synchronized, better brands of radio clocks are normally accurate to the second.

Typical radio "atomic clocks" require placement in a location with a relatively unobstructed atmospheric path to the transmitter, perform synchronization once a day during the nighttime, and need fair to good atmospheric conditions to successfully update the time. The device that keeps track of the time between, or without, updates is usually a cheap and relatively inaccurate quartz-crystal clock, since it is thought that an expensive precise time keeper is not necessary with automatic atomic clock updates. The clock may include an indicator to alert users to possible inaccuracy when synchronization has not been successful within the last 24 to 48 hours.

See also

• Network Time Protocol
• NIST-F1
• Optical Atomic Clock [2]
• Radio clock
• Second
• Télé Distribution Française
• GPS
• Optical Clockwork

References

External links

• PTB Braunschweig, Germany - with link in English language
• National Physical Laboratory (UK) time website
• NIST Internet Time Service (ITS): Set Your Computer Clock Via the Internet
• NIST press release about chip-scaled atomic clock
• NIST website
• Web pages on atomic clocks by The Science Museum (London)

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